The present study aimed to find out dynamic changes of apoptosis in human cytomegalovirus (HCMV) infected cells and the influence of HCMV infection on activation of caspase3 and the expression of apoptosis-regulating genes, bcl-2 and fas mRNA. The sequential changes of apoptotic cell rate in high and low MOI (MOI= 2.5 and 0.25 respectively) of HCMV infected human embryonic lung fibroblasts (HELFs) at 1 h, 12 h, 24 h, 36 h, 48 h, 72 h and 96 h post-infection were measured by flow cytometry. The expression levels of caspase-3 protein and bcl-2 and fas mRNA in HCMV infected cells (MOI=0.25) at 72 h post-infection were detected by West blot and in situ hybridization methods, respectively. It was found that the ratio of apoptotic cells in normal controls was consistently lower, but the rates in low and high MOI infected cells were gradually increased with time prolonged, reached peak at 96 h (8.85 %) and 72 h (25.63 %), respectively. By West blot analysis, only a narrow band of 32 kD (1 kD=0. 992 1 ku) procaspase-3 was found in normal cells, but a wider procaspase-3 band and a much wider band of 17 kD proteins (p17) appeared in the infected cells. Meanwhile, the expression of bcl-2 mRNA was higher and that of fas mRNA was lower in the normal HELF cells, whereas there were significantly lower bcl-2 mRNA and higher fas mRNA expression levels in HCMV infected cells. It was concluded that HCMV was a stronger inducer of apoptosis in HELF cells. Caspase-3, as the marker of undergoing apoptosis,was expressed increasingly and activated in the infected cells, indicating its action in HCMV-inducing apoptosis. Down-regulating bcl-2 mRNA expression and up-regulating fas mRNA expression were also involved in the mechanism of HCMV-induced apoptosis.